1. Field of the Invention
The present invention relates to an improvement of a circuit for clamping a DC level of a video signal including a synchronizing signal at a prescribed potential.
2. Description of the Background Art
FIG. 18 is a schematic block diagram showing an example of a conventional television receiver. FIG. 19 is a waveform diagram of a composite video signal.
For simplicity of drawing, an antenna, high frequency circuitry and so on of the television receiver are not shown in FIG. 18. Referring to FIG. 18, the television receiver includes an input terminal 1 to which a composite video signal is input, a feedback type clamp circuit 100, a video amplifying circuit 101, a video output circuit 102, a CRT 103, a color signal circuit 104, a synchronizing deflection circuit 105, and a clamp pulse generating circuit 106. The composite video signal input to the input terminal 1 has its DC level clamped at a prescribed level by the clamp circuit 100, and thereafter it is applied to the video amplifying circuit 101. The video amplifying circuit 101 amplifies a color signal after separation of color subcarrier and the synchronizing signal. The color signal circuit 104 forms a color difference signal from the color subcarrier. The video output circuit 102 adds the color difference signal to a luminance signal to form a signal to be applied to three electrodes, that is, the R, G and B electrodes of a CRT 103. The sync deflection circuit 105 separates the synchronizing signal, synchronizes vertical and horizontal oscillations and generates a signal to form an image. The clamp pulse generating circuit 106 generates a clamp pulse during a certain DC level period of the composite video signal, in response to the synchronizing signal which has been subjected to amplitude separation by the sync deflection circuit 105. Generally, the clamp pulse is inserted to the back porch of the composite video signal shown in FIG. 19.
FIG. 20 is a block diagram showing an example of such a clamp circuit. The clamp circuit shown in FIG. 20 includes an input terminal 1 to which a composite video signal is input, a level shift circuit 2 for clamping the DC level of the composite video signal at a prescribed potential, an output terminal 3 from which the composite video signal clamped by the level shift circuit 2 is output, a clamp pulse input terminal 5 to which the clamp pulse is input, a constant voltage source 6 generating a constant DC voltage, a comparator 4 for comparing the output from the level shift circuit 2 and the voltage from the constant voltage source 6 in response to a clamp pulse, and a capacitor 7 charging/discharging of which is controlled by the comparator 4.
In operation, the composite video signal is applied through the input terminal 1 to the level shift circuit 2. The level shift circuit 2 adds a certain potential to the composite video signal to shift the level of the composite video signal, the composite video signal having its level shifted is output from the output terminal 3. The potential applied to the composite video signal is to make constant the DC level of the composite video signal which fluctuates. The method for generating the potential to be added is as follows. The comparator 4 compares the level of the composite video signal from the level shift circuit 2 with the potential of the constant voltage source 6 connected to the comparator 4 in response to a clamp pulse from the clamp pulse input terminal 5, and generates a difference current corresponding to the difference between the DC level of the composite video signal and the potential of the constant voltage source 6. The period in which the clamp pulse is input corresponds to the period of the DC level in the composite video signal, which is generally the period of the back porch. The generated difference current is output to the level shift circuit 2. By this difference current, the capacitor 7 connected between the level shift circuit 2 and the ground terminal is charged or discharged, and voltages at both ends of the capacitor 7 are maintained until the next clamp pulse is input. By the voltages at both ends of the capacitor 7 provided in this manner, the amount of level shift by the level shift circuit 2 is controlled. By these operations, a certain DC level of the composite video signal is corrected to be equal to the potential of the constant voltage source 6.
High definition television systems (HDTV) have come to be commercially available very recently.
FIG. 21 is a waveform diagram of a video signal including a three value synchronizing signal (which is a proposed standard synchronizing signal for realizing high vision broadcasting) such as a HDTV video signal. In FIG. 21, the solid line represents the original waveform on the transmitting side, while the dotted line represents the waveform at the receiving end (corresponding to the input terminal 1 of FIG. 20) such as a VTR reproduction, which waveform is a little moderate because of transmission loss through the transmission path.
In the above described circuitry, a clamp pulse forming circuit must be separately provided in addition to the clamp circuit, resulting in a large scale circuitry. In the composite video signal reproduced by a VTR or the like, cross talk from adjacent tracks, leaked FM carrier and other noise are output superposed on the video signal Further, in the weak electric field such as in the case of receiving television broadcasting, there is caused degradation of S/N ratio.
If there is, for example, a noise at a certain DC level portion of the input composite video signal, accurate level comparison cannot be done, and as a result, accurate clamping operation becomes impossible. It is possible to remove the color burst signal successive to the synchronizing signal by means of a low pass filter and setting the position of the clamp pulse at that portion However, by this method, the color burst cannot be completely removed, and therefore level comparison is not very accurate.
Such problem similarly occurs in the video signals for HDTV. Further, the DC level period of the video signal is quite short in the HDTV video signal, and precision in level detection is not sufficient. Alternatively, the three value synchronizing signal portion may be integrated by an integrating circuit for providing an average and the position of the clamp pulse may be set at that portion. However, the waveform of the three value synchronizing signal cannot be completely removed, and the level comparison is not very accurate. Clamping using negative peak value (bottom level of the synchronizing pulse of negative polarity) in the video signal is known as another method. However, accurate clamping is not achieved because of the influence of noise or degradation of amplitude characteristic dependent on conditions of the transmission path.